Environmental concerns have led to continued efforts to reduce the CO, hydrocarbon and nitrogen oxide (NOx) emissions of compression ignited (diesel-fueled) and spark ignited (gasoline-fueled) light duty internal combustion engines. Further, there have been continued efforts to reduce the particulate emissions of compression ignited internal combustion engines. To meet the upcoming emission standards for heavy duty diesel vehicles, original equipment manufacturers (OEMs) will rely on the use of additional exhaust gas after-treatment devices. Such exhaust gas after-treatment devices may include catalytic converters, which can contain one or more oxidation catalysts, NOx storage catalysts, and/or NH3 reduction catalysts; and/or a particulate trap.
Oxidation catalysts can become poisoned and rendered less effective by exposure to certain elements/compounds present in engine exhaust gasses, particularly by exposure to phosphorus and phosphorus compounds introduced into the exhaust gas by the degradation of phosphorus-containing lubricating oil additives. Reduction catalysts are sensitive to sulfur and sulfur compounds in the engine exhaust gas introduced by the degradation of both the base oil used to blend the lubricant, and sulfur-containing lubricating oil additives. Particulate traps can become blocked by metallic ash, which is a product of degraded metal-containing lubricating oil additives.
To insure a long service life, lubricating oil additives that exert a minimum negative impact on such after-treatment devices must be identified, and OEM specifications for “new service fill” ad “first fill” heavy duty diesel (HDD) lubricants require maximum sulfur levels of 0.4 mass %; maximum phosphorus levels of 0.12 mass %, and sulfated ash contents below 1.1 mass %, which lubricants are referred to as “mid-SAPS” lubricants (where “SAPS” is an acronym for “Sulfated Ash, Phosphorus, Sulfur”). In the future, OEMs may further restrict these levels maximum levels to 0.08 mass % phosphorus, 0.2 mass % sulfur and 0.8 mass % sulfated ash, with such lubricants being referred to as “low-SAPS” lubricating oil compositions.
As the amounts of phosphorus, sulfur and ash-containing lubricant additives are being reduced to provide mid- and low-SAPS lubricants that are compatible with exhaust gas after-treatment devices, the lubricating oil composition must continue to provide the high levels of lubricant performance, including adequate detergency, dictated by the “new service”, and “first fill” specifications of the OEM's, such as the ACEA E6 and MB p228.51 specifications for heavy duty engine lubricants.
United States patent application number US 2005/0043191 discloses a lubricating oil formulation which is free of zinc and phosphorus and comprises at least one borated dispersant, a mixture of metal detergents, an amine antioxidant and a trinuclear molybdenum additive. Provided the composition comprises at least 700 ppm boron and at least 80 ppm molybdenum, acceptable engine performance is achieved.
One of the most effective antioxidant and antiwear agents, from both a performance and cost-effectiveness standpoint, used conventionally in lubricating oil compositions for internal combustion engines comprises dihydrocarbyl dithiophosphate metal salts. The metal may be an alkali or alkaline earth metal, or aluminum, lead, tin, molybdenum, manganese, nickel or copper. Of these, zinc salts of dihydrocarbyl dithiophosphate (ZUDP) are most commonly used. While such compounds are particularly effective antioxidants and antiwear agents and inexpensive, such compounds introduce phosphorus, sulfur and ash into the engine that can shorten the service life of exhaust gas after-treatment devices, as described supra. All metal-containing lubricant additives contribute to the ash content of the lubricant and in addition to ZDDP, a significant amount of lubricant ash is introduced by metal-based detergent additives. Such metal-based detergents include two distinct components, “soap”, the function of which is to remove deposits from engine parts, particularly piston deposits; and overbasing, which neutralizes acidic combustion products. Each of the soap and overbasing components of the detergent contribute to the ash content of the detergent.
There are three classes of detergents used conventionally in the formulation of lubricating oil compositions for the lubrication of internal combustion engine crankcases, specifically metal salts of carboxylates (e.g., salicylates), phenates and sulfonates. Carboxylate soap is generally considered to provide superior piston cleaning performance and as superior performance allows for the use of less detergent soap, carboxylate detergents have been favored in the formulation of mid- and low-SAPS lubricating oil compositions. Carboxylates also provide an antioxidancy credit and do not contribute to copper corrosivity. However, carboxylate detergents are available from only a few sources and therefore, supply is constrained. Phenate and sulfonate detergents each have performance debits and credits relative to one another. Phenates, for example, provide a credit in antioxidancy relative sulfonates, but have a deleterious effect on copper corrosivity. Sulfonates provide an antiwear credit relative to phenates, but introduce more sulfur and do not boost antioxidancy. Therefore, when used, phenate and sulfonate detergents are commonly employed in combination. It would be beneficial to be able to provide mid- and low-SAPS lubricating oil compositions formulated with phenate/sulfonate mixtures, which provide acceptable piston cleanliness performance.
The present inventors have identified an anomaly in the performance of phenate detergents. Specifically, while increasing the amount of phenate soap had not been found to have a significant effect on piston cleanliness performance in lubricating oil compositions having conventional ash contents, it has been observed that piston cleanliness performance can be improved dramatically by increasing the level of phenate soap in mid- and low-SAPS lubricating oil compositions therefore allowing formulators to adjust the phenate/sulfonate detergent mixture to provide excellent piston cleanliness performance in mid- and low-SAPS lubricating oil compositions.